Beam current integrator



1955 R. DE LIBAN ETAL BEAM CURRENT INTEGRATOR Filed Oct. 18, 1948 2 Sheets-Sheet l BEAM CURREA/ T //v Tia/PA 70A H0} 775/? SUPPLY Accac'm mva VOL TA 65 5UPPL V w 3145 ml INVENTORS. ROBERT DE L/aAA/ WILLIAM A. BAKER 25, 1955 R. DE LIBAN ET AL BEAM CURRENT INTEGRATOR 2 Sheets-Sheet 2 Filed Oct. 18, 1948 QB INVENTORS.

ROBERT DEL/BAN WILL/AM R BAKER United States Patent BEAlVI CURRENT INTEGRATOR Robert De Liban and William R. Baker, Berkeley, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Application flctober 18, 1948, Serial No. 55,148

6'Claims. (Cl. 235-92) This invention relates generally to mass separators of the calutron type and more particularly to an improved method of an apparatus for integrating the beam current of a mass separator.

Mass separators areprincipally employed in separating the isotopes 'of the elements. They are especially-useful for the separation of macroscopic amounts 'of any mixture of'isotopes which maybe-ionized.

In general, a mass separator has a'source for vaporizing'a quantity of material containingan element whose isotopesit is desired to separate. A portion of 'the'vapor 'i's ionizedby an electric arc causing'ions of the several isotopesto be produced. These ionized particles are accelerated and formed into a beam by the accelerating electrodes; and this ion beam is projected into a magnetic field. Here the ions-follow curved paths,the radii of' curvature of thepaths being proportional to thesquare 'roots of the masses of the ions. Thus the ions are con- 'centrated in accordance with their-masses and are then collected and de-ionized, thereby producing a-deposit'of the element enriched withthe selected isotope.

Beam integrators arenot new; in fact, ever since'cyclotrons were developed to the point where there was enough beam current to 'detect,-some sort of devicewasnecessary -to measure thequantity ofbeam current with respect to time. All of these circuits dealt with currentsin' the order of micro-amperes and, when the mass separator'was developed 'with its beam current "inthe milliampere region, it was necessary todevelop a different circuit thatwould function in this higher current range.

Simple integrators for this purpose havebeen built and employed. 'These first integrators were very simple, merelya low leakage condenser'shunted'by a'neon glow lamp, the combination being placed in-series with the current path to the collector elements. -Beam current flowed into the condenser 'whichbecame charged to the strikingvoltage of the neon lamp which, upon firing, discharged the condenser aftenthe fashion ofthe conventional relaxation oscillator. A'rnechanical register'for counting the number of discharges completed the "device; the num- "ber "of counts-being the intergral of the beamcurrent over'a given'period of time. "However; for' stable opera- *tion' of 'a'mass separator it is'necessary to maintain the potential of the collector electrodes at arreven level, which in the preferred itypeof mass separator isator near ground ipotential. .Thisfirst integrating device did'not'fulfill this requirementto any satisfactory degree. However, other "models ofintegratorsasjfor example,.'those shown in the copending .applicationLofIWilliam RfiBaker, Serial No. .45,607,.Lfiled August 23, l948,.were. later developed incorporating single vacuumtubes ofthe triode or pentode type wherein the gain ofthe tube was utilized tokeep the collector;,potentialionwat more even zlevel and nearer to ground potential.

The invention hereinafter describedlovercomeslthe:difiiculties of the earlier integrating circuits by utilizing the scgain of'iamultistage*amplifier to keep the collector voltice age variations much smaller in magnitude than was the case of the earlier models of beam current integrators.

It is therefore an object of this invention to provide an improved method of and apparatus for integrating the beam current in a mass separator without interfering with the operation thereof.

Another object of this invention is to provide in a beam current integrator a substantially zero collector element potential.

Still another object of this invention is to provide an improved apparatus for determining the quantity of beam current that flows during a given time in a mass separator.

A still further object of this invention is to provide an apparatus for maintaining the potential of the collecting electrodes constant while obtaining the summation of the beam current over a given time interval.

Other objects and advantages of this invention will be apparent to those skilled in the arts to which it pertains, upon consideration of the following specifications and drawings, in which:

Figure 1 shows a plan view in diagrammatic form of the essential parts of a calutron including the beam current integrator in block form;

Fig. 2 is a schematic diagram of the fundamental circuit of an integrator for illustrative purposes; and

Fig. 3 is a schematic diagram of the complete beam current integrator employed in the present invention.

Referring now more particularly to Fig. 1, it will be seen that ions produced by an ion source 1 are drawn through an ion exit slit 2 by the accelerating force of the potential applied to the ion accelerating electrodes 3. The ions so accelerated are formedinto a beam in passing through a collimating slit d between the accelerating electrodes 3, and are acted upon by a magnetic field extending vertically from the plane of the drawing and defined by the pole face 6. The path of the ions is curved by the magnetic field toward a collector element 7 which is connected through the beam current integrator 8 to ground.

For the purpose of illustrating the operation, a simplified circuit of an integrator is shown in Fig. 2. It will be seen that the beam current to be integrated flows in at terminal 9, through the series circuit comprising a battery 10, a condenser 11, and a triode 12. Triode 12 has its cathode connected to ground .13 through a conductor 14. Connected in shunt with the condenser 11 is the series circuit comprising the primary winding of a transformer 16 and a gaseous discharge device 17. The secondary winding of the transformer 16 is directly connected to an electromechanical register 18. The grid of triode 12 is connected through a bias battery 19 to the input terminal 9.

The operation of this simplified circuit will now be con sidered. The beam current flowing in at terminal 9 is stored as a charge onlcondenser 11. When the voltage on the condenser 11 reaches the firing potential of the gaseous discharge tube 17, the gaseous discharge tube conducts and thereby discharges the condenser 11. The discharge path includes the primary winding of transformer 16; and thedischarge of condenser 11 produces a pulse in the secondary winding of the transformer 16 of sufficient amplitude to operate the electromechanicalregister 18.

It is evident that the entire beam-current flows through the plate impedance of the triode 12, or more correctly, as positive charge flows into one side of condenser 11 electrons flow from the cathode to the anode of triode 12 and these electrons flow into the other side of condenser 11 to balance the positive current flowing in. The cathode of triode 12 is returned directly to ground through conductor 14. The effective impedance of the mass separator, is. therefore connected between the input terminal 9 and ground 13 of the integrator. This impedance is quite high, as apparatus of this type tends to act as a constant current generator.

During operation, the grid must attain a potential in the operating range of the tube such that the net cathode to control grid voltage will allow a plate current to flow which is just equal to the beam current of the mass separator. Thus the control grid potential is inversely proportional to the gain of the vacuum tube. In this manner the potential of the collector element 7 (Fig. 1) which is connected to the control grid of triode 12 is maintained at a potential near that of ground; and this potential varies by the factor of 1 divided by the amplification factor of the change in beam current.

It will be noted that the arrangement of the components of this simplified integrator is novel in that the minus terminal of the plate supply battery 10 is returned directly to the beam collector terminal 9. Thus the plate current of triode 12 is at all times equal to the beam current. If no beam current flows there is likewise an absolute cutofi in the plate current of triode 12 and positively no count could be recorded by the register 18. Now, if one milliampere of beam current flows through the circuit, the same current appears everywhere in the circuit and the only variable quantity is the bias voltage that appears on the grid of triode 12; that is, exactly enough grid bias appears to allow one milliampere of plate current to fiow at whatever the efiective plate voltage is. The term unity-current voltage amplifier could appropriately be applied to the foregoing arrangement of components.

The range of this combination would be between the extremes of (l) absolute zero beam current, and (2) whatever current that would bring the grid of the triode to the Zero bias level. Naturally, at an excessive current level, the grid would be forced positive in order to allow that much current to flow and the resulting grid current to ground would be lost current that would not flow through the condenser 11 and be recorded. As long as the control grid of the triode 12 works in a minus region, the tube constants other than the amplification factor have no importance at all. Any source of nonlinearity is then confined to the glow lamp-condenser combination not always firing at the same voltage or to poor collector insulation. This latter problem becomes less and less as the circuit input is made to run close to ground potential.

The purpose of the bias battery is to make the collector operate at near zero voltage with respect to ground and at the same time bias the tube to cutofi in case the collector becomes short-circuited to ground.

Fig. 3 shows an improved beam current integrator wherein the high gain of a multistage amplifier is used to maintain the collector voltage very stable and near to ground potential.

For practical purposes the battery supplies shown in Fig. 2 have been replaced with an alternating current supply which is of conventional full wave design.

This full wave power supply 21 comprises a power transformer 22, a rectifier tube 23 for positive voltage, a pi section filter including the condensers 24 and 25, and a filter choke 26. The gaseous regulator tubes 28 and 29 and their series-dropping resistor 31 are connected between the positive unregulated voltage terminal 32 and the center tap of the power transformer 22. The junction between the gaseous regulator tubes 28 and 29 is connected to ground 33 through a conductor 34. This arrangement provides a source of regulated positive voltage obtainable at the terminal 36 and also a source of regulated negative voltage obtainable at the terminal 37. An additional rectifier tube 38 for supplying unregulated negative voltage obtainable at the terminal 39, supplies pulsating direct current to the pi section filter comprising the condensers 41 and 42 and the filter choke 43; the output of this filter is available at the terminal 39. The resistors 44 and 45 which are connected in series between the regulated negative voltage supply terminal 37 and ground 33 form a voltage divider for the regulated negative volt-- age output. The junction between the resistors 44 and 45 is connected through a resistor 47 to the control grid of a pulse amplifier triode tube 48 for supplying grid bias; thereto. This pulse amplifier triode 48 is used to operate: the electromechanical counter 49 which is connected be-- tween the anode of triode 48 and the unregulated positive: voltage terminal 32. The cathode of triode 48 is con-- nected to ground 33 by conductor 51. The grid of the: pulse amplifier tube 48 is connected to the junction between the integrating condenser 52, the cathode of at strobotron discharge tube 53, and the anode of a pentode 54, by means of a resistor 56 and a condenser 57 connected in series.

The strobotron discharge tube 53 has its grids connected to ground 33 through a grid current limiting resis-- tor 58; and its anode is connected to the remaining side: of the integrating condenser 52 through a current limit-- ing resistor 59. The junction between the current limiting resistor 59 and the integrating condenser 52 is connected to an input terminal 61 and also to the control grid of a pentode amplifier tube 62 through a coupling network comprising a resistor 63 shunted by a condenser 64.

There is a voltage dividing network, comprising a resistor 66 and a potentiometer 67 in series, connected from the regulated positive voltage terminal 36 to ground 33. The movable contact of the potentiometer 67 is connected to the cathode of the pentode 62 for setting the operating voltage of this pentode. The screen grid of pentode 62 is also connected to the regulated positive voltage terminal 36 by a conductor 68; and the anode receives its operating voltage through the plate load resistor 69, which is connected to the unregulated positive voltage terminal 32.

The signal voltage present at the anode of the pentode 62 is transferred to the control grid of a triode amplifier tube 71 through a resistor 72 shunted by a condenser 73. This triode amplifier tube 71 has its cathode connected to ground 33 by conductor 74, and its control grid is connected through a resistor 76 to the unregulated negative voltage terminal 39 from which the control grid receives its negative operating bias voltage. The anode voltage for the triode amplifier tube 71 is obtained from the unregulated positive voltage terminal 32 through the anode load resistor 77. The network comprising a resistor 78 shunted by a condenser 79 is connected between the anode of the triode 71 and the control grid of the pentode 54 for transferring signal voltage therebetween. Grid bias voltage from the unregulated negative voltage terminal 39 is supplied to the control grid of the pentode 54 through the grid resistor 81. The pentode 54 has its cathode connected through a conductor 82 to the regulated negative terminal 37 and its screen grid is connected to ground 33 through a conductor 83. The ground 33 is brought out to a terminal 84 for connection to the apparatus with which the integrator is to be used. The input terminal 61 is by-passed with a by-pass condenser 86 to the grounded terminal 84.

In this improved circuit as in the simplified circuit of Fig. 2 the collector element of the calutron is connected to the input terminal 61 of the beam current integrator. The beam current flows in at the input terminal 61 and into the integrating condenser 52; at the same time the pentode 54 passes a charging current which is equal and opposite to the beam current. The effect of these two events is to charge the condenser 52 to the firing potential of the strobotron tube 53, whereupon the strobotron 53 fires and discharges the condenser 52. This charge-discharge cycle repeats as long as beam current continues to flow into the integrator.

The pulse amplifier triode 48 is normally biased to cutofi by the negative voltage applied to its control grid from the voltage divider comprising resistors 44 and 45; when the strobotron tube 53 fires and discharges the condenser 52, a steep positive pulse is applied to the control grid through the condenser 57 and the resistor 56; this causes a'pulse of platecurrent"to""flow,*-andthe"electro mechanical register 49 operatesya'dding one'more'coun on the dial. Thus the summation of counts over a given interval of time is the integral of the beam-current.

This integrating circuit uses a novel arrangement of components; that is, the pentode'-54has its anode connected through the integrating condenser 52 to the input 61 and its cathode connected to the negative terminal 37 of the power supply-21, while the junction between gaseous regulator tubes 28' and 29 is grounded. Thus it is evident that the plate current of'pentode 54 is at all times equal in magnitude to the'beam current, and if there is no beam current there is likewise no count recorded on the electromechanicalregister. The only variable quantity is the control grid-cathode voltage of pentode 54.

The voltage thatappears between the control grid and the cathode'of the pentode '54 is a function of the negative direct current bias voltage supplied from terminal 39 of the power supply and the signal voltage supplied from the anode of the triode amplifier tube 71. The signal voltage applied to the control grid.of pentode 54 is the product of the gain of the'triode 71 times the gain of pentode 62 times the amplitude of the voltage present between the input terminal 61 and the. ground terminal 84. These variations incollector voltage which appear between the input terminal 61 and, ground are applied to the control grid of thefirst amplifier pentode 62 through the coupling network comprising the resistor 63 and the condenser 64 in shunt. This typeof. network, the impedance of which is given by the formula,

where [Z] is the absolute magnitude of the impedance w=21rf f=frequency in cycles per second C =capacity of the coupling condenser C=capacity of the coupling condenser in farads R=resistance of the coupling resistor in ohms plifier, then the rising response characteristic of the coupling network when added to the drooping response characteristic of the vacuum tube -amplifier,,produces a response curve which is substantially constant over a much greater frequency range than *would be the case 'of the vacuum tube amplifier alone. 'In this: manner%an amplifier is produced which is stableunder a'rwide range of operating conditions; and which has a substantially constant gain over a wide frequency range.

Thus, through the use of this stable negative feedback amplifier, any voltage variations of the collector element 7 (Fig. 1) are decreased in the following manner. These voltage variations are coupled to the control grid of the first amplifier pentode tube 62, amplified and then passed on to the control grid of a triode amplifier tube 71 through a coupling network similar to the one described above and which in this instance comprises a resistor 72 shunted by a condenser 73. This amplified signal voltage is now transferred to the control grid of the pentode tube 54, through another coupling network of the above described type comprising a resistor 78 shunted by a condenser 79, and is applied in parallel with the direct current bias voltage to this control grid of the pentode tube 54 through which the beam current flows as the condenser 52 is charged.

Here as in the simplified circuit the control grid of pentode-54= automatically assumes *the' correct'bias v'oltage' to allow the beam current to flow. In this 'improvedcircuit, however, thereisa multistage amplifier connected between the extremes of (1) absolute zero beam current and (2) whatever value of current that will drive the control grid of the pentode 62 to the Zero bias level. Naturally, at an excessive current level, the control grid will be driven positive in order to allow this much current to flow and the resulting grid current to ground will be lost current that will not flow through the integrating condenser '52. and be recorded. As long as the control grid of the pentode 62 operates on the negative part of its characteristic curve the tube constants other than the amplification factor have little importance. Any source of nonlinearity is then confined to the glow lamp-condenser combination not always firing at the same voltage or to poor collector insulation. This latter problem becomes less and less as the circuit input or the collector element is made to run closer and closer to the ground potential.

The cathode bias on the pentode 62 is made variable and can be set at any desired value by changing the position of the movable armof potentiometer 67. By making this bias variable, it is possible to make the collector run at any voltage with respect to ground that is desired. This follows from the feature brought out previously that the bias on the grid automatically sets itself at the value necessary to allow the beam'current to flow; therefore, if a voltage equal to the required bias is inserted in between the collector terminal and the grid of the tube or inserted in the cathode circuit then the actual collector voltage has to run at zero with respect to ground. If more injected bias voltage is used, then the collector has to run at a high enough positive voltage to cancel out the excess voltage; and conversely, if less injected bias voltage is used, then the collector has to run negatively enough to cancel the superfiuity of voltage.

Although we have described our invention with respect to a particular embodiment thereof, it is not limited to this embodiment nor otherwise, except by the terms of the followingclaims.

What is claimed is:

'1. In an integrating device having aninput, the combination comprising a series circuit including a condenser,

a variable electronic impedance, anda source of power, -said condenser having one side connected to said input andthe otherside connected to one terminal of said impedance, said source of power being connected between ground and a second terminal of said impedance whereby said condenser becomes charged with electrical energy through said series circuit in response to the current flowing at said input, a gaseous discharge device connected in shunt with said condenser for discharging said condenser at a predetermined energy level, a pulse amplifier tube having at least a cathode, an anode, and a control grid, said cathode being connected to ground, said control grid being connected to said condenser, said anode being connected to one side of an electromechanical register and the other side of said register being connected to said source of power, said pulse amplifier tube serving to actuate said register in response to voltage pulses received at the control grid of said pulse amplifier tube as an indication of the quantity of current flowing at said input, and a high gain multistage amplifier connected between said input and said variable electronic impedance whereby signal voltage present at said input is amplified and applied to said electronic impedance so as to vary the impedance thereof, said variations in impedance serving to maintain the voltage at said input substantially constant and substantially zero in magnitude with respect to ground potential.

2. In an integrating device having an input, the combination comprising a series circuit including a condenser, a source of power and a vacuum tube containing at least a cathode, an anode, and a control grid, said condenser having one side connected to said input and the other side connected to the anode of said vacuum tube, said source of power being connected between ground and the cathode of said vacuum tube whereby said condenser becomes charged with electrical energy through said series circuit in response to the current flowing at said input, a gaseous discharge device connected in shunt with said condenser for discharging said condenser at a predetermined energy level, a pulse amplifier tube having at least a cathode, an anode, and a control grid, said cathode being connected to ground, said control grid being connected to said condenser and said anode being connected to one side of an electromechanical register, the other side of said register being connected to said source of power whereby said pulse amplifier tube actuates said register in response to voltage pulses received at the control grid of said pulse amplifier tube as an indication of the quantity of current flowing at said input, and a high gain negative feedback amplifier connected between said input and the control grid of the vacuum tube in said series circuit whereby signal voltage present at said input is amplified and fed back to the control grid of said series vacuum tube so as to vary the current flow therethrough, said variations in current flow serving to maintain the voltage at said input substantially constant and substantially zero with respect to ground potential.

3. In an integrating device having an input, the combination comprising a series circuit including a condenser, a pentode tube and a source of power, said condenser having one side connected to said input and the other side connected to the anode of said pentode, said source of power being connected between ground and the cathode of said pentode whereby said condenser becomescharged with electrical energy through said series circuit in response to the current flowing at said input, a strobotron tube connected in shunt with said condenser for discharging said condenser at a predetermined energy level, a pulse amplifier tube having at least a cathode, an anode and a control grid, said cathode being connected to ground, said control grid being connected to said condenser, and said anode being connected to one side of an electromechanical register, the other side of said register being connected to said source of power whereby said pulse amplifier tube actuates said register in response to voltage pulses received at the grid of said pulse tube as an indication of the quantity of current flowing at said input, and a high gain multistage amplifier connected between said input and the control grid of the pentode in said series circuit for amplifying signal voltage present at said input, said amplified signal being fed back to the control grid of said pentode tube, thereby controlling the current flowing in said series circuit, variations in said current flow serving to maintain the voltage at said input substantially constant and substantially Zero in magnitude with respect to ground potential.

4. In an integrating device having an input, the combination comprising a condenser for storing energy connected to said input, a variable impedance connected to said condenser, a source of power having a negative terminal thereof connected to said variable impedance, means connected across said condenser for discharging said condenser at a predetermined energy level, an electromechanical register having one terminal connected to a positive terminal of said source of power, means connected between a second terminal of said register and said discharge means for actuating said register each time said condenser discharges, and an amplifier connected between said input and said variable impedance for varying the impedance thereof.

5. In combination with a mass separator having a collector element, a condenser for storing energy connected to said collector, a triode vacuum tube having the anode thereof connected to said condenser, a source of power having a negative terminal thereof connected to the cathode of said tube, means connected across said condenser for discharging said condenser at a predetermined energy level, an electromechanical register having one terminal connected to a positive terminal of said source of power, means connected between a second terminal of said register and said discharge means for actuating said register each time said condenser discharges, and an amplifier connected between said input and the control grid of said tube for varying the impedance thereof.

6. In a current integrating device having an input, the combination comprising a condenser for storing energy connected to said input, a triode vacuum tube having the anode thereof connected to said condenser, a source of power having a negative terminal thereof connected to the cathode of said tube, a strobotron tube connected in parallel with said condenser for discharging said condenser at a predetermined energy level, an electromechanical register having one terminal connected to a positive terminal of said source of power, means connected between a second terminal of said register and said strobotron tube for actuating said register each time said condenser discharges, and a multistage amplifier having its input connected to said device input and its output connected to the control grid of said tube for varying the impedance thereof.

References Cited in the file of this patent UNITED STATES PATENTS 

